1. Field of the Invention
The present invention relates generally to rotary shaft sealing and, more particularly, is concerned with an adaptable sealing mechanism useful for providing zero emission leakage around the shaft in a wide variety of valves.
2. Description of the Background
In the valve industry, one of the major problems encountered is leakage of process product to the atmosphere through the stem area. Stem seals begin to leak over time due to a number of reasons including compression set, loss of energization, contamination from the milieu, compound aging, and elastomer compound breakdown.
Leakage at some minute level is always exhibited through any packing and is becoming an ever increasing concern and expense from the environmental regulatory standpoint. State and Federal regulations have in the past required BACT (best available control technology) standards on new sources of regulated emissions and RACT (reasonably available control technology) standards on existing sources. The regulatory standards have changed a great deal over the past decades and more recently generally require greater reductions of emissions with less regard to economic costs required for conformity.
Previous patents have addressed various environmental concerns in the area of shaft seals. U.S. Pat. No. 3,979,104 to B. L. La Coste shows a method for controlling leakage of radioactive steam along the stems of valves used in nuclear power plants. The La Coste device utilizes a flexible double bellows sealing arrangement with a seal chamber disposed between the contaminated steam and a low pressurized gland condenser. A buffer layer of uncontaminated steam having a pressure higher than the pressure of the contaminated steam is introduced into the seal chamber through a conduit to maintain zero leakage of contaminated steam into the low pressure gland condenser. For general use in controlling valve stem leakage, this device appears to have a number of serious problems. It is designed to be built into the valve housing so that it is of little value for the many valves that are already in operation. The seal is somewhat bulky and complicated to be useful for inclusion in many new valves. It is suitable only with a steam or gaseous sealing medium. Furthermore, if the seal fails, there appears to be little available warning of the failure and there is no provision for additional contingency sealing when the fault is detected.
U.S. Pat. No. 4,972,867 to J. O. Ruesch shows a housing for placement over the valve stem area of a valve. This housing contains a back-up seal and sensors to provide a warning that leakage past the primary seal has occurred. Additionally the Ruesch device includes a drain tube from the housing so leakage can be stored rather than spilled. The Ruesch device provides no means for interacting with the primary seal of the valve to improve the original primary sealing in any way. Also, should the Ruesch device secondary seal leak prior to the time when repair is effected, there appears to be no way to prevent emission leakage.
U.S. Pat. No. 4,537,385 to D. D. Brag uses inner and outer metal bellows to transmit the pressure of the controlled product behind the primary seal to produce a near zero pressure differential across the primary seal thereby allowing the primary seal elements to float so as to have little wear. The main pressure differential is across the secondary seal. This valve is used in compliance with EPA standards for VOC (volatile organic compound) emissions but is somewhat bulky and complicated for general use with a wide variety of valves.
Various other patents, such as U.S. Pat. No. 4,364,542 to Meyer, provide means for injecting highly viscous fluids including semi-fluid plastic material for actuating sealing elements for seals which generally depend on the viscous nature of the semi-fluid material itself as a seal. These seals provide no way to eliminate all leakage and there is no mechanism for providing a warning if the seals should begin to leak.
Consequently, a need exists for improvements in rotary shaft seals which will result in greater versatility, reliability and dependability of operation at reduced levels of capital investment.